Dichroic polarizers are absorptive, linear polarizers having a vectoral anisotropy in the absorption of incident light. The polarizer, therefore, has the property of differential absorption (and transmission) of the components of an incident beam of light depending on the direction of vibration of the components. Generally, the polarizer will transmit radiant energy along one electromagnetic vector and absorb energy along a perpendicular electromagnetic vector. A beam of incident light, on entering the dichroic polarizer, encounters two different absorption coefficients, one low and one high so that the emergent light vibrates substantially in the direction of low absorption (high transmission).
The development of synthetic polarizers has made possible the widespread utility of light-polarizing elements for a wide variety of applications, such as in liquid crystal display screens in which crossed polarizers are used in conjunction with an addressable liquid crystal material to provide the basis for image formation. Polarizers have also been used in many optical applications, such as to reducing glare or the brightness of specular reflection in photography or CRT monitors to reduce glare.
Among the known synthetic polarizers are “K-type” polarizers in which the linear dichroic light polarizing materials are prepared by dehydration of poly(vinyl alcohol). K-type polarizers may also be known as inherent polarizers since the absorbing chromophore is the result of conjugation in the polymer backbone, rather than due to dyes added to the polymer matrix. These polarizers comprise a sheet of oriented poly(vinyl alcohol) having light polarizing (dichroic) molecules of poly(acetylene) blocks (i.e. —[CH═CH—]n In formed by heating the oriented poly(vinyl alcohol) sheet in the presence of a dehydration catalyst such as vapors of aqueous hydrochloric acid. By orienting the poly(vinyl alcohol) matrix uniaxially the transition moments of the chromophores, the conjugated poly(acetylene) blocks, are also oriented and the material becomes visibly dichroic.
While K-type polarizers can be made by conventional acid processes, these processes necessarily involving the handling of, and potential exposure to, hazardous quantities of acid, usually hydrochloric acid. Additionally, the vapor-phase acid processes can result in non-uniform catalytic dehydration, which can lead to streaking or mottling of the polarizer, rendering it unsuitable for many precision optical applications. See, for example U.S. Pat. No. 5,773,834 (Kadaba et al.). Hence, there is a need for a process for preparing K-type polarizers that does not use large quantities of hazardous and corrosive acids (such as HCl vapors) to effect dehydration and can produce high quality, uniform polarizers.